1. Field of the Invention
This invention is directed to circuit breakers and, more particularly, to non-automatic power circuit breakers which employ a trip mechanism.
2. Background Information
The most important application feature that distinguishes a power circuit breaker from a molded case circuit breaker is the ability of the power circuit breaker to tolerate very high overcurrent levels without tripping. The maximum current level that a power circuit breaker can tolerate for a short time period without internal damage is called its short time withstand current rating (e.g., generally, short time withstand current ratings of 0.5, 1.0 and 3.0 seconds are established). The short time withstand current rating identifies the mechanical and thermal ability of the power circuit breaker to withstand overcurrents for the given period of time and is specified as a characteristic of the breaker independent of the current levels at which the trip functions are actuated.
Power circuit breakers are typically used in radial distribution systems to feed a load center, motor control center or panel boards. A multiplicity of circuit breakers in these load centers then feed a variety of individual loads. To coordinate the tripping characteristics of the power circuit breaker with these downstream breakers, it is very desirable to design the mechanical characteristics of the breaker so that its xe2x80x9cwithstand currentxe2x80x9d level is as high as possible, preferably equal to the available fault current from the source of supply.
If power circuit breakers are applied within the limits of their short time withstand current rating, they are generally applied without an instantaneous trip feature. These power circuit breakers can tolerate any available fault current for that short time and give the downstream circuit breakers an ample opportunity to clear any fault that may develop on one of the load lines. Only if the fault is located immediately downstream, with no intervening circuit breaker, should the power circuit breaker ultimately trip open. For any other fault location, the power circuit breaker should stay closed. Thus, continuity of service is preserved for all of the feeder loads that are not directly involved with the fault. This feature, where only the circuit breaker immediately upstream from any fault opens, is called xe2x80x9cselective coordinationxe2x80x9d or xe2x80x9cselectivity.xe2x80x9d
Modern low voltage power circuit breakers are frequently designed with sufficient energy in the closing springs to close on a modest overcurrent condition, but with insufficient energy to close on a relatively large fault current condition. Such circuit breakers are designed to xe2x80x9cwithstandxe2x80x9d such fault currents once closed, but need not be able to xe2x80x9cclose and latchxe2x80x9d on them.
This inability to xe2x80x9cclose and latchxe2x80x9d results from a conscious design compromise between the closing spring energy of the operating mechanism and the resulting operating endurance and reliability. If such a circuit breaker attempts to close on a relatively large fault current, then closure will normally be incomplete and damage may result to the separable contacts if this condition is permitted to persist. Integral trip units installed in such circuit breakers can detect such a fault current condition upon closure and issue an immediate trip signal if the fault current exceeds a xe2x80x9cclose and latchxe2x80x9d threshold for the particular circuit breaker. This important self-protection feature is commonly termed xe2x80x9cmaking current releasexe2x80x9d (MCR) and is employed by many modern low voltage power circuit breakers.
Typically, the major cost elements of an integral trip unit are the current sensors (e.g., one current sensor per phase) which sense the phase currents that flow through the circuit breaker, and the circuitry that processes the sensed current signals to determine when and if the circuit breaker should be tripped to open the separable contacts. The cost of these elements is typically several hundred dollars. However, once these elements are present, the MCR functionality is gained at minimal incremental cost.
There is a segment of the power circuit breaker market that applies power circuit breakers as simple switches requiring no automatic overcurrent protection and, thus, no overcurrent trip unit. Alternatively, such switches, which are termed non-automatic circuit breakers, may be sold to users that prefer to provide their own overcurrent protection system.
In this instance, the manufacturer assigns to the non-automatic circuit breaker a short circuit current rating which is equal to the withstand capability. However, since there is no trip unit, the MCR function cannot be provided and the inability of the circuit breaker to close completely onto a fault current that is otherwise within its short circuit rating makes it vulnerable to misuse. It, therefore, becomes desirable to satisfy the MCR function by an alternate, albeit less expensive, approach. Accordingly, there is room for improvement.
This need and others are satisfied by the invention, which is directed to a non-automatic power circuit breaker which provides the protection of a making current release (MCR) function therein, but without the expense of a complex multi-function trip unit.
The non-automatic power circuit breaker comprises separable contact means for movement between a closed position and an open position, and operating means for moving the separable contact means between the open and closed positions. A means determines a signal having a first state for the open position of the separable contact means and a second state after the separable contact means moves to the closed position. A sensing means senses an electrical current which passes through the separable contact means and provides a signal corresponding to the electrical current. A tripping means employs the signal corresponding to the electrical current and the signal corresponding to the open and closed positions of the separable contact means for providing a trip signal. The tripping means includes means for disabling the trip signal when the signal of the means for determining has the second state. A trip actuator means employs the trip signal for actuating the operating means to move the separable contact means to the open position thereof. A means powers the trip actuator means from the signal of the sensing means.
Preferably, the sensing means includes a current sensor having an output, and the means for powering the trip actuator means includes a capacitor, and means for charging the capacitor from the output of the current sensor, with the capacitor powering the trip actuator means.
The means for determining preferably includes means cooperating with the operating means for determining the open and closed positions of the separable contact means, and switch means driven by the means cooperating with the operating means for determining the signal having the first and second states. As a further refinement, the switch means includes a switch having a first position corresponding to the first state and a second position corresponding to the second state, and the means cooperating with the operating means includes means for delaying movement of the switch from the first position to the second position thereof a predetermined time after the separable contact means moves to the closed position thereof.
As another aspect of the invention, a non-automatic power circuit breaker comprises separable contact means for movement between a closed position and an open position, and operating means for moving the separable contact means between the open and closed positions. A means determines a signal having a first state for the open position of the separable contact means, and having a second state a predetermined time after the separable contact means moves to the closed position thereof. A sensing means senses the electrical current of the separable contact means and includes a coil having an air core or a low permeability core for providing a signal corresponding to the electrical current. An analog tripping means employs the signal corresponding to the electrical current and the signal corresponding to the open and closed positions of the separable contact means for providing a trip signal. The analog tripping means includes means for disabling the trip signal after the separable contact means moves to the closed position thereof when the signal of the means for determining has the second state. A trip actuator means employs the trip signal for actuating the operating means to move the separable contact means to the open position thereof.
As a further aspect of the invention, a non-automatic power circuit breaker comprises separable contact means for movement between a closed position and an open position, and operating means for moving the separable contact means between the open and closed positions thereof. A means cooperates with the operating means for providing a signal having a first state for the open position of the separable contact means, and having a second state a predetermined time after the separable contact means moves to the closed position thereof. A current sensor includes a coil having an air core or a low permeability core. The current sensor senses electrical current flowing in the separable contact means and provides a signal corresponding to the electrical current. A low energy trip actuator employs a trip signal for actuating the operating means to move the separable contact means to the open position thereof. An analog trip means comprises a capacitor charged by the signal of the current sensor, means employing the signal of the current sensor for providing the trip signal, means disabling the trip signal when the signal of the means cooperating with the operating means has the second state, and means employing the capacitor for powering the trip actuator means.